Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers.
Personal audio devices may employ integrated circuits that may include many components, and each of the components may operate from and/or be synchronized with a clock signal. When the clock signal becomes unavailable, the components may fail to operate and/or fall out of synchronization. The integrated circuits then begin producing erroneous output and eventually cease operating. For example, when the output of one component is communicated to the input of another component, the two components may be synchronized such that the second component knows when the first component has output data intended for reading by the second component.
A phase-locked loop (PLL) is one component that operates from a clock signal. The PLL may receive an input clock signal and generate a clock signal that may be used to operate and/or synchronize other components. In particular, a PLL generates an output clock with a phase and/or frequency that has a determined relationship to an input clock's phase and frequency, such as the master clock's phase and frequency. The output clock signal from the PLL may be used to operate, for example, audio components such as an audio amplifier.
A PLL may be further enhanced with a redundant uninterruptable clock conditioning circuit. Such a clock conditioning circuit may be configured to, in the absence of a master clock to the PLL, generate a substitute clock signal such that the clock signal is uninterrupted in the absence of the master clock, thus allowing redundant operation of the PLL even when no reference clock input signal is present.